This invention relates to a fuel-air mixer for a combustor of a gas turbine engine and more particularly to a means of reducing the formation of carbon resin on such a fuel-air mixer.
Gas turbine engines generally comprise a compressor for pressurizing air and a combustor for mixing and burning fuel with the air. The heated mixture is then flowed into a turbine to generate power. Fuel that is burned is normally premixed with air prior to undergoing combustion in order to minimize smoke and other undesirable by-products and to maximize the efficiency of the combustion process.
A fuel-air mixer is designed to atomize the fuel and to premix it with air in order to produce efficient and complete combustion. Low pressure fuel-air mixers have been designed which incorporate primary and secondary counterrotational air swirlers which atomize fuel by the high shear forces developed in the area or zone of interaction between the counterrotational flows. These air swirlers are typically known as swirler cups and comprise annular air chambers having tangential air jets formed therein. The tangential air jets swirl the air prior to intermixing with the fuel to enhance atomization as well as mixing.
A very common problem with fuel-air mixers is the formation of carbon resin and/or coke, commonly referred to as carboning. Both carbon resin and coke consist of a build up of carbon caused by unburned fuel being heated at nonstoichiometric conditions on hot surfaces. Carbon resin is typically made up of eighty percent carbon and 20 percent oxygen with traces of nitrogen and sulfur. Carbon resin is typically formed at temperatures between 300 and 900xc2x0 F. On the other hand, coke is nearly 100% carbon and is formed at temperatures in excess of 1000xc2x0 F. The surface temperature of a typical fuel-air mixer exposed to fuel and compressor discharge air during operation is less than 1000xc2x0 F. making it more prone to carbon resin formation than coke formation. This formation of carbon resin leads to build up which clogs passageways resulting in degradation of engine operation and in some cases, severe engine damage. For industrial gas turbines, which burn heavy diesel fuels, the potential for carbon resin formation on fuel-air mixers is greater. Heavy diesel fuels contain heavy hydrocarbons with long evaporation times which results in longer dwell times on wetted surfaces. Extensive carboning can result in obstructed venturi passages blocking the flow of fuel. This condition impairs engine performance and can cause fuel to collect at low points in the combustor case, ultimately igniting, and burning through the case.
The present invention is directed to a fuel-air mixer for the combustor of a gas turbine engine that reduces carboning in the venturi of the fuel-air mixer.
The fuel-air mixer comprises a substantially annular venturi defined by a wall having an inner surface and a longitudinal axis therethrough and includes an upstream end and a downstream end. A primary swirler having an annular upstream end and a downstream end located adjacent to the upstream end of the venturi, includes a plurality of primary air jets interposed between the upstream and downstream ends of the primary swirler. Airflow supplied to the primary air jets is swirled and subsequently delivered to the venturi. A fuel nozzle, positioned inside the upstream end of the primary swirler and adjacent to the venturi, includes a fuel passage therein through which fuel is sprayed into the venturi at a designated spray angle. Purge airflow circumscribing the fuel passage flows substantially parallel to the longitudinal axis of the venturi providing a boundary layer of air along the inner surface of the venturi. The boundary layer of air minimizes the amount of fuel contacting the inner surface of the venturi subsequently reducing carboning.